Add the ability to add a note to a thread

[ojanvafai]

A couple use cases:

• For needs filter threads, lets you jot down what filter you want to setup
• For must-do/urgent/backlog/blocked, let's you write what action needs taking for that thread so you don't have to reread the thread to remember why you needed to do more work on it.
• If we have the note override the subject line in the threadlist, then this doubles as a way of making the threadlist rows not be stuck with the subject line of the original message. Git-style, we can make the first line of the note be what is shown as the subject.